Movement of the tropics altered human history during the last 800 years

The new research suggests the expansion of the tropics can help explain the collapse of the Ottoman empire, the decline of China's Ming Dynasty and the abandonment of Jamestown Colony.

The abandonment of the Jamestown Colony was at least partially caused by droughts triggered by the northward expansion of the tropics. Photo by National Park Service

Oct. 15 (UPI) -- According to a new survey, the first-of-its-kind, the northern boundary of the tropics has fluctuated over the course of the last 800 years, affecting the positioning of the Sonoran, Mohave and Saharan deserts.

Thirty degrees north and south of the equator are generally designated as the boundary lines of the tropics. But the tropics aren't stationary. The tropics are defined by precipitation, and as climate patterns change, so do precipitation trends.

The new study, published Monday in the journal Nature Geoscience, proved the tropics' northern boundary has moved as much as four degrees north and south of the northern 30th parallel during the last 800 years.

An international team of scientists analyzed tree rings in five locations in the Northern Hemisphere, revealing a history of regional precipitation regimes. The analysis allowed scientists to trace the movement of the tropical belt's northern boundary between 1203 and 2003.

According to the data, the tropics expanded north between 1568 and 1634. During the same period, extreme droughts altered human history. And the Ottoman empire collapsed, China's Ming Dynasty ended and the Jamestown Colony was abandoned.

"Our results suggest that climate change was one of the contributing factors to those societal disruptions," Valerie Trouet, researcher at the University of Arizona, said in a news release.

Researchers were also able to draw connections between historic volcanic eruptions and changes in the tropical boundaries. In 1815, Indonesia's Tambora volcano sent large volumes of ash in the atmosphere, dramatically cooling global temperatures. In Europe, 1816 was known as the "year without a summer."

Authors of the new study hope their analysis will help climate scientists better understand how climate change will impact the movement of tropical boundaries. Until now, scientists had only traced the movement of the tropics back to the early 20th century.

"This is the first reconstruction that went back to pre-industrial times," Trouet said. "To know what the natural climate variability is, we need to go farther back in time than the last 150 years."